The invention relates to a system, by which the proportion of ferromagnetic particles in a dielectric medium is measured. The primary object is to monitor the purity of lubricating oil in respect of the particles which have got into it in consequence of metal wearing. The measurement system includes both the method and arrangement for realizing it.
As known, the lubricating oil is a necessary medium in the gears, where the metal surfaces move in respect of each other, to reduce friction and to prevent excessive heating up. Such gears are i.a. different motors, power trains and actuators, and for example rolling lines, process lines of paper machines and printing machines as well as machines in the power plants. The order of magnitude of the lubricating oil's consumption in the world is ten million cubic meters per annum.
During use the lubricating oil degrades because of contamination, and eventually it is unsuitable for its aim. The particles loosen from a gear constitute the most of the contaminants, as much as 95%. The proportion of such particles in the lubricating oil is the best indicator of the gear wear condition. For this reason the information about the wear particle proportion is very useful, when it is striven to avoid failure of a gear and the costs caused by it.
At least the following methods can in principle be used for the measurement of the wear particle proportion:                Ferrography, in which an oil sample is driven through a static magnetic field, in which case the iron particles with different size move different distances. The result is examined by a special microscope. The measuring equipment is relatively expensive and susceptible to external interferences.        Emission spectroscopy, in which an oil sample is for example heated strongly, and thereafter the spectrum of the radiation, which comes from the sample, is measured. As known, different materials, like iron, radiate at different frequencies and are seen in the spectrum. The measuring equipment is relatively expensive.        Neutron activation analysis, in which an oil sample is bombarded by neutrons so that the wear particles in it become radioactive for a time. The proportion of the particles can be deduced from the radiation which they transmit. The method is accurate, but it requires an expensive measuring equipment and is slow.        Optical method, in which the scattering of the light from an oil sample is examined. The measuring equipment is expensive and is not valid, when the oil is dark.        The method of the linear magnetic response, in which ferromagnetic particles in an oil sample are magnetized in the linear range of the magnetization curve. The impedance of the coil, by which the magnetization is implemented, rises a little when the amount of the ferromagnetic material increases. This impedance is converted into an output quantity which then in principle gives the particle proportion. A flaw of the method is its unsatisfactory sensitivity; when the particle proportion is low, the measurement signal does not stand out in the noise.        The method of the nonlinear magnetic response, in which the inductive coupling between the sending and receiving elements belonging to the measuring equipment is changed by saturating magnetically the ferromagnetic particles in an oil sample. The measurement signal, the level of which depends on the particle proportion, is obtained by processing the receiving signal.        
The last-mentioned method is applied in the invention, which method has proved most suitable for most of the practical requirements. Let us consider the method first in the theory level: FIG. 1 shows the very well known magnetization curve of the ferromagnetic materials in HB coordinates. Quantity H is the strength of the magnetic field, which falls from outside into a ferromagnetic material, and quantity B is the magnetic flux density in said material. When quantity H starts to rise from zero, quantity B rises first linearly and relatively steeply when the ferromagnetic material polarizes step by step to the direction of the magnetic field. After the polarization has completed, or the material has saturated magnetically, the magnetic flux density B rises only very gently, when quantity H still is strengthened. The ratio B/H is the permeability p, or magnetic conductivity. If a ferromagnetic object is a part of a magnetic circuit, which comprises a primary and secondary winding, the mutual inductance and coupling between the windings depend on the permeability. When the signal level is low, the coupling coefficient is constant, but when using higher signal levels it decreases, if the ferromagnetic material is now and then in saturation. The derivative dB/dH of the curve, or the dynamic permeability μd, gives the magnetic conductivity, when the magnetic field strength H fluctuates in a relatively narrow range near an operating point. In FIG. 1 the operating point WP1 is in the linear range, where the dynamic permeability is relatively high. The operating point WP2 again is in the nonlinear range, where the dynamic permeability is low. Let us assume that a ferromagnetic material is magnetized to a certain operating point and an alternating current with relatively low constant amplitude is fed to the primary. Then, in the case of said operating point WP2 a clearly lower alternating voltage induces to the secondary winding than in the case of the operating point WP1 because of weakening of the coupling.
FIG. 2 shows as a block diagram a system, known from the publication WO 2004/077044, for measuring the proportion of ferromagnetic particles, which system utilizes the nonlinear magnetic response. The system comprises a low frequency source G1 and an excitating source G2, a magnetizing winding 211, an excitating winding 212 and a secondary winding 220, a first 230 and second 240 detector as well as a frequency multiplier 250. The medium LO, the particle proportion of which is measured, is in a dielectric vessel in the middle of the windings.
The low frequency source G1 generates a sinusoidal feed FD, the frequency f1 of which is in the range of 50-100 Hz, and the excitating source G2 generates a sinusoidal excitation EX, the frequency f2 of which is in the range of 10-100 kHz. The current of the low frequency source is led to the magnetizing winding 211, and the magnetic flux arising in this winding flows through the medium LO and secondary winding 220. The magnetic field of the magnetizing winding is so strong that the ferromagnetic particles in the medium saturate during its peaks. The current of the excitating source G2 is led to the excitating winding 212, and the magnetic flux arising in this winding flows through the medium LO and secondary winding as well, being summed with the magnetic flux of the magnetizing winding. The magnetic field strength of the excitating winding is at least one order lower than the magnetic field strength of the magnetizing winding by amplitude.
The secondary winding 220 outputs the response RE, which is detected coherently in the first detector 230 by using the excitation EX as a ‘carrier’. The detecting result SD1 of the response is detected coherently in the second detector 240 by using a ‘carrier’, the frequency of which is 2·f1. This subcarrier is generated from the feed FD by the frequency multiplier 250. The second detecting result, or the output signal SD2, shows then the amplitude of the signal SD1, more exactly the amplitude of the component with the frequency 2·f1. In principle, also envelope detectors could be used as detectors, but in that case the signal-to-noise ratio in the output signal would be poorer than when using coherent detectors. In both cases filters belong to the detectors, by which filters excess parts are removed from the signal spectra.
FIG. 3a shows the response RE in the time domain, its principled waveform. The response RE is a sum of two response components. The first response component RE1 is the voltage induced in the secondary winding by the feed FD. Because of the saturation of the ferromagnetic particles the first response component is not sinusoidal but flattened at its peaks. The second response component RE2 is the voltage induced in the secondary winding by the excitation EX. Its amplitude depends on the dynamic permeability μd≠ΔB/ΔH. When the feed FD is close to the zero level corresponding to the first operating point WP1 in FIG. 1, μd and the second response component RE2 are relatively high. When the feed FD is in the peak range corresponding to the second operating point WP2 in FIG. 1, μd and the second response component are relatively low.
FIG. 3b shows the detecting result SD1 of the response in the time domain. It follows the fluctuation of the amplitude of the second response component RE2 by shape. The fundamental frequency of the signal SD1 is 2·f1, because the second response component has a minimum at both the positive and negative peak of the feed FD and a maximum at each zero point.
As mentioned, the output signal SD2 is detected from the signal SD1 using a subcarrier with the frequency 2·f1. The filtered detecting result is a straight line in the time domain. If the medium were pure of the ferromagnetic particles, it would be magnetically linear, in which case the level of the output signal SD2 would be zero. When the particle proportion increases, also the level of the output signal rises. This dependence is linear. The magnetizing feed FD has a certain optimum amplitude in respect of the quality of the output signal.
FIG. 4a shows the response RE in the frequency domain, its principled spectrum. The first response component RE1 causes to the spectrum a frequency component at point f1 and odd harmonics of this frequency component. The second response component RE2 causes to the spectrum a frequency component at point f2 and on its each side a sideband, in which the spacing between the frequency components is 2f1. These sidebands as well as the harmonics of the component with the frequency f1 are naturally a result of the distortion in the response RE due to the magnetic non-linearity of the medium.
FIG. 4b shows the detecting result SD1 of the response in the frequency domain. Because the correctly phased excitation EX is used as the subcarrier, the result is a baseband signal corresponding to said sidebands, the fundamental frequency of which signal is 2f1. The spectrum of the output signal SD2 detected from the signal SD1 is then a mere DC component after the low-pass filtering.
The above-described method applies two sinusoidal feeds with different frequencies. The term ‘two-frequency method’ is used for it and other corresponding ones. They are selective to the ferromagnetic wearing particles, and e.g. the oil darkness level has no meaning. A drawback of the two-frequency methods is that their accuracy is not sufficient in an environment, where there are interfering fields. Such circumstances may prevail in an industry plant and moving vehicles. Here it must be noticed that FIGS. 3a-4b show the matter very ideally. In practice the amount of the ferromagnetic material in a medium is very slight, in which case the actual signal corresponding to the excitation is in danger of vanishing into the interferences and noise. Another serious drawback is that the stray capacitance between the windings causes in the secondary winding a voltage with frequency f2, the amplitude of which is high, for example one thousand times the amplitude of the voltage being caused by the magnetic non-linearity. The elimination of this kind of interfering voltage, which is called parasitic voltage in this description, from the measuring result is difficult.